1. Field of the Invention
The present invention relates to an image pickup apparatus such as an electronic camera or the like and, more particularly, to an image pickup apparatus having an auto-focusing function and a control method of such an apparatus.
2. Description of the Related Art
The Official Gazette of Japanese Patent Application Laid-Open No. H04-267211 discloses a solid-state image pickup apparatus in which an image pickup element constructed by two-dimensionally arranging a number of microlenses and pixels in each of which a relative position of a photoelectric conversion unit is deviated with respect to an optical axis of the microlens also has a function of a focus detection element. In the solid-state image pickup apparatus of Japanese Patent Application Laid-Open No. H04-267211, in a case where a normal photographing is performed, an image is generated by adding signals of the pixels whose relative deviation directions to the microlens differ. In a case where a focus position of a photographing lens is calculated, by executing a correlation operation by using a pair of image signals generated respectively by pixel lines in which the relative deviation directions of the photoelectric conversion units to the microlens are different from each other, the focus position of the photographing lens is calculated.
At the time of photographing, there is a case where what is called vignetting in which a part of a light flux which is irradiated toward a group of focus detecting pixels is shielded by a photographing optical system (including optical elements such as lenses, iris, and the like and a lens barrel for holding them) occurs. The occurrence of the vignetting causes an unevenness of intensity of the image signals due to a decrease in light amount (unevenness of light receiving sensitivity of each focus detecting pixel: hereinbelow, referred to as “shading”) in at least one of the pair of generated image signals. There is a case where such a decrease in image signal level and a shading occurring due to the vignetting reduce a degree of coincidence of the pair of generated image signals and decrease a precision of focus detection.
To prevent such a problem, therefore, the Official Gazette of Japanese Patent Application Laid-Open No. 2011-114553 discloses such a technique that a shading of a pair of image signals obtained from a pixel group for receiving light fluxes passing through different exit pupil areas of a photographing optical system is corrected based on image pickup pixel signals.
The Official Gazette of Japanese Patent Application Laid-Open No. 2008-252522 also discloses such a technique that plots of an image height and a distorted aberration amount of an image pickup element are stored and an approximate function is derived from their values, thereby performing a distortion crosswise difference correction in which a necessary amount of data is suppressed.
In order to perform a good focus detection, it is necessary to perform the correction of the shading at a high precision. However, there is such a problem that if the shading correction is performed at a high precision, an operation scale increases and it takes a time for an operation processing.